1. Field of the Invention
The present invention relates to a reduction gear apparatus capable of obtaining a high speed reducing ratio in a single stage, a drive apparatus using this reduction gear apparatus, and an optical system such as a photographic lens for driving (e.g., zooming) a lens using this drive apparatus or a camera on which this photographic lens is mounted.
2. Related Background Art
A photographic lens is constituted by a plurality of lens groups and performs adjustment such as focusing or zooming of the lens by moving these lens groups in the optical axis direction. In particular, in the photographic lens mounted on a camera, the lens groups are moved by turning an operation ring mounted on the outer surface of the lens. A photographer selects a mode using a powered/manual selection knob upon judging the photographic situation or performs powered or manual driving of the operation ring.
Powered driving is performed by a drive unit mounted in a lens barrel. The drive unit incorporates various switches, a drive motor, a power transmission mechanism, and the like.
Powered driving of the lens is performed by operating a switch lever arranged on the drive unit. Manual driving is performed by directly operating the operation ring with a hand.
In a mechanism for transmitting rotation from such a drive source to the load side, a torque limiter is generally arranged in a transmission system to cause slippage so as not to adversely affect the drive source when a large force acts on the load side or from the load side.
The torque limiter mechanism is arranged in the power transmission unit. In particular, in driving a lens to an operation end at high speed, the torque limiter mechanism has an effect to absorb an impact force acting on tooth flanks when the lens is stopped. A clutch mechanism for performing powered or manual driving is also arranged. FIGS. 18 and 19 show a photographic lens, and particularly, a zoom power transmission mechanism.
FIG. 18 shows a power transmission route in powered driving of a lens, and FIG. 19 shows a case wherein an operation ring is manually driven.
Referring to FIG. 18, members except for a lens barrel and the operation lens are incorporated in a drive unit.
Referring to FIGS. 18 and 19, a lens operation ring 55 has meshing teeth 55a on its circumferential surface. A stepped sleeve 58 is mounted on an output shaft 57 extending from the case of a reduction gear apparatus 56 formed integrally with a motor 66. The sleeve 58 is fixed to the output shaft 57 by, e.g., a pin 59, so that the sleeve 58 rotates together with the output shaft 57.
An output gear 60 of the reduction gear apparatus 56 is rotatably mounted on the sleeve 58 and normally biased against the motor 66 side by a spring 62 through a slide washer 61. This structure serves as a torque limiter in which the output gear 60 of the reduction gear apparatus 56 and the output shaft 57 of the reduction gear apparatus 56 rotate together but slip with respect to the sleeve 58 of the reduction gear apparatus 56, thereby reducing the force acting on the teeth and hence preventing damage to the teeth.
This torque limiter mechanism enhances the effect of absorbing the impact force acting on the tooth flanks particularly when the lens is driven to the operation end at high speed and stopped.
An intermediate gear 63 is interposed between the output gear 60 of the reduction gear apparatus 56 and the operation ring 55. The intermediate gear 63 is rotatable with respect to a shaft 64, and at the same time slidable along the shaft 64. Meshing teeth 63a formed on the outer circumferential surface of the intermediate gear 63 mesh with the output gear 60 of the reduction gear apparatus 56 and the operation ring 55. Using this power transmission mechanism, when the photographer operates a predetermined switch arranged on the drive unit, a rotation power generated by the motor 66 is transmitted to the operation ring 55 through the output gear 60 of the reduction gear apparatus 56 and the intermediate gear 63, thereby performing powered driving of the lens.
FIG. 19 shows the case in which the operation ring is manually driven. In this case, when the photographer operates a clutch lever (not shown), a clutch plate 65 is rotated. A selection pin 66 extending on the clutch plate 65 engages with the intermediate gear 63, and the intermediate gear 63 slides along the shaft 64 during the operation of the clutch lever. The intermediate gear 63 then disengages from the output gear 60 of the reduction gear apparatus 56. In manually driving the lens, the selection lever is operated as described to cut off the power transmission path from the drive motor. The lens is driven by directly manually operating the operation ring or operating a lever attached to the operation ring.
In selectively switching between powered driving and manual driving, meshing teeth must be generally formed on the outer circumferential surface, as can be represented by a spur gear in a system wherein the intermediate gear interposed between the output gear 60 of the reduction gear apparatus 56 and the operation ring 55 to mesh therewith is displaced to engage or disengage the meshing teeth.
The power transmission member for transmitting power through gear meshing and the torque limiter member for limiting the transmission torque in overloaded rotation to prevent damage to the tooth flanks are independently arranged as separate members in the power transmission mechanism for coupling the motor to the operation ring of the lens.
The following problems are posed in the above reduction gear apparatus.
In the clutch mechanism, one shaft and a gear train are added between the driving side and the driven side, and an extra space for these members is required. In addition, a clearance must be assured between the clutch gear and the shaft to allow rotation or sliding of the clutch gear. This increases the backlash together with the addition of the gear train.
On the other hand, in the torque limiter mechanism, since the sleeve is attached to the distal end of the rotating shaft of the motor serving as a drive source through a straight pin or a spring pin, the motor gear is prone to run out depending on conditions such as the fitting length of the motor gear with the shaft and the backlash of the pin. In addition, the area of the frictional surface cannot be sufficiently assured depending on the relationship between the shaft and the gear. Due to an increase in the biasing force of a friction spring, an unnecessary force acts to make the friction torque unstable. This also results in a spatial disadvantage.
When a lens or the like is to be driven using such a drive apparatus, the lens cannot be stably driven.